Safety switch



May 6, 1941. H. T. sPARRow SAFETY SWITCH Filed July 8, 1958 immunmN :inventor Htmbnfl '1I'. Sparrow Cmorncg Patented May 6, 1941 SAFETY SWITCH Hubert T. Sparrow, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application July 8, 1938, Serial No. 218,140

'l Claims.

This invention relates generally .to the art of switching, and more particularly to that type of switch which-is operated by a heat responsive timer and which must be manually reset.

Such a switch has particular utility in the automatic control of oil burners and will be de- .scribed in this connection although it is clear furnace and also to energize the ignition means for producing a spark or ilashing a gas pilot to ignite the oil so delivered. If combustion occurs the system will remain in running position until the main control instrument is satisfied, at which time it will shut the system down. If for some reason, however, the oil does not ignite, it is not desirable to continue to deliver raw fuel to the furnace. Therefore a timer is employed to shut the system down a predetermined time after it has started, unless combustion has taken place in the meantime. When combustion occurs a switch is operated which stops operation of the timer, but if combustion does not occur the timer opens a switch, shutting the system down, and it is required that this switch be manually reset, at which time the trouble causing failure of com- ,bustion may be corrected.

It is therefore an object oflthis invention to provide a thermally timed switch which will open a circuit a predetermined time after it has been closed, providing the action of the timer has not been-stopped in the meantime.

- A further object is to provide a magnet for holding a switch in one position and a time controlled means for overcoming the eiect of the magnet on the switch so that a biasing means can actuate the switch.

Another object is to provide a magnet for I holding a switch in one position and a thermocouple for neutralizing the effect of said magnet to permit a biasing means to actuate the switch. n

Another object is to provide a manual means for resetting the lever but which is incapable of actuating the switch unless the magnet can hold the lever.

Still another object is to provide a pivoted switch operating lever which carries a permanent .magnet at one end and which cooperates with a core to normally hold `the switch in closed positionVbut which is'released for actuation by a spring when a thermocouple neutralizes the effect of the magnet.

Still 'another object is to provide an adjustable means in a device of the above type for limiting the movement of the magnet toward the core, and also a means for adjusting the biasing spring.

These and other objects will readily become apparent to those skilled in the art as the following specifcation is read in the light lof the 'accompanying drawing, in the single figure of which my invention is shown as applied to a simple control system for an oil burner.

Referring to the safety switch of this invention, it will be seen that the entire structure is carried by means of a base i2. Suitably connected to the base i2 is a supporting arm I3 to whichjs pivoted as indicated at I4 a lever I5. A tension spring i6 is connected to the lever I5 just above the pivot I4 and also to the base I2 by means 'of the adjustable screw 3I for the purpose oi biasing the lever i5 for rotation in a clockwise direction. Suitably connected to the lower end of the lever I5 is a bar magnet I1 having two poles indicated as N and S. Also supported on the vbase I2 is a U-shaped core generally indicated at I8 and having two legs I9 and 20. The end of leg 20 is adapted to lie adjacentthe N pole of the bar magnet Hand the leg I9 is adapted to lie adjacent the S pole of the bar magnet I1. Suitably located in the general proximity o the above mechanism is a metallic block 2I with which is associated an electric heater 22 for the purpose of heating the block. A'thermocouple generally indicated at 23 and having a hot junction 24 and a cold junction 25 is positioned to respond to the temperature of the metallic block 2|. One end of the cold junction 25 is electrically connected by means of a conductor V26 to one end of a winding 21 which surrounds the leg 20 of the core I8. The other end of the winding 21 is connected by means of the conductor 28 to one end of a second winding 29 which is located around the leg I9 of the core I8. The other end of the winding 29 is connected by means of a. conductor 30 to the other side of the cold junction 25 of thermocouple 23.

The core I8 is not permanently magnetized and there is a magnetic attraction between the legs I9 and 2li of core I8 and the bar magnet I'I. When the metallic block 2l is cold, both the cold and hot junctions of the thermocouple 23 will ybe at the same temperature and no current will be owing in the circuit connected to the thermocouple. If the heater 22 is now energized. it will to raise the temperature ci the block il .rence the hot junction lil ci the thermocouiole i3, which will develop an electromotive .force causing a flow ci current in the direction lcated by the arrow. It will ice noted that with the current flow in this direction the Wind- "l .is so applied tothe il) as to magnetico C, in such a manner that a north pole ls proLL y`iced -adj acent the N pole of the bar magnet il, and the winding 219 is so applied to the les i3 as to produce a south pole adjacent the S pole the bar magnet l'I. the temperature oi :metallic block 2l increases, the `temperature diiference between 'the hot and cold lunctlcns the thermocouple 23 will increase and hence the current flow in the thermocouple circuit will increase and will eventually attain a value wherein the magnetic ilux produced by the windings 21 and 29 is substantially equal and opposite to the magnetic flux produced by the bar *magnet I1. At this 'time there will be insui cient attraction between the core I8 and the magnet I'I to keep the biasing spring i6 from rotating the lever I5 in a. clockwise direction. The extent of this rotation under the influence of the spring I6 is such that even though the metallic block 2| should now cool to low ternn oerature and the current flow through the windl ings 21 and 29 be reduced to raero, the distance between the core I8 and bar magnet Il will be so great that the magnetic attraction at that distance cannot rotate the lever I 5 against the action of the spring I6. However, if the lever l5 were to be manually returned to its original position the gap between the core i8 and magnet I1 would be reduced to the point where the magnetic attraction between the two would be sufficient to hold the lever I5 in that position.

The base I2 is provided with a screw-threaded hole 35 in which is screwed the stern 36. The stem 36 is provided with a knurled head 3'! and extends through an oversized hole 38 in the core i8. Rotation of the head 31 will move the stern 3F in and out of the core i8 in order that the air gap between the permanent magnet I1 and the core I8 may be readily adjusted. Such an adjustment will vary the maximum attractive force between the permanent magnet I1 and the core I8 and hence will vary the point at which the biasing spring I6 will open the switch. Another way to vary this point is to adjust the screw 3I to vary the force exerted by the spring IG on the lever I5.

Also connected to the base I2 is a support 39 to which is pivoted as at #D a reset member generally indicated at 4I. The reset member ll is provided with a resilient member I2 which is in the general form of a Ll and which carries at its free end a stationary contact 43 which is adapted to cooperate with a movable contact #d located at the upper end of the lever I5. A tens-inn spring 45 is connected between the reset member 4I and the support 39 i'or the purpose of normally holding the member ll adjacent e .raton 4S on the support 39. With the lever llt in the position shown in the drawing, wherein the har magnet I1 is ariacent the core I8, the contacts 43 and 44 will be in engagement. The arm l2 oi' the reset member 4I is resilient and regardless of the air gap between the bar magnet and the core I8 as determined by the adjustable stem 36, will always maintain its contact 43 in engagement with the movable contact H. the bar magnet I1 -is released by the current flowing through the windings ill and 2B as deuw FLL titl

Til

l ld will ne brealr lthe cold contact lil.

scribed alcove le 'the lever lli in a cloche of the lever it member fil, at

"le spring l5 will direction until t ages an arm ted. l lever cannot now be moved cuocaA the action oi the magnet l'i but mi x ually reset. .llor this purpose a ha i lle LH :is provided on the reset member il to rotate it 1n a counter-clockwise direction, causing the arm fit to cani. the lever l5 to its original r ositlon wherein the bar magnet il lies adjacent the core iii. if at this time the temperature of the blcclr 2l been :reduced to the point where the current rlow through the windings 4321 and te is so low that the 'flux produced thereby cannot overcome the effect of the magnet I1, the lever l5 will remain in lts set position. If, however, the current is still oi sufficient strength to nullify the effect of magnet I1, the ,spring I6 will return the lever l5 to its clockwise position as soon as the reset handle t9 is released. It will be noted that as the reset handle 49 is moved to the left the stationary contact 43 is moved to the left also so 'that 'the contacts I3 and Ml cannot be closed unless the magnet I1 is holding the lever l5 in its vertical. position and until the handle ll@ is released to rnove the stationary contact 43 again to the "ght and into engagement with the movable c llt. The purpose oi this arrangement provide a device wherein it will be i e to manually maintain the circuit closed wh he safety device is calling for an open circuit condition.

Although the above mechanism may be readily adapted for various different uses, one of the moet obvious being an overload cut-out, it will be described in connection with a simple matic control system for an oil burner. primary winding lill ol a step-down trans-ll generally indicated at 5l and having a sec winding itt lecl by means oi" conductors M troni line wires and 5t. The usual room thermostat is indicated generally at lil and conlprises a bimetalllc element 58 operating a resilient contact blade 59 and a rigid contact blade 6U. Controlled by the thermostat lil is relay. winding lill which operates two switch arms lill and 62. The usual oil burner motor is indicated at t3 and the ignition means at lili. IZlhe cembre sponslve switches are indicated gener at and are oi usual form wherein a the nelly responsive element is positioned to respon existence or non-existence oi combustio the furnace combustion chamber. ,The nernlal element is connected by means of a slip friction connection to the switch. arm tt so that t .glu t6 will be actuated not upon the attenant., any predetermined temperature but rather upon a chance in direction of temperature variation. 'The switch is shown in its cold position engaging the contact B1 and as the temperature at the thermally responsive element increases the arm 66 will rst engage the hot contact B8 and then is the thermally rcsoonsive element continues it will also rotate the mercury switch tt to lnealr the circ"it therethrough. 0n e. decrease in tempel-m ture at the thermal elerlffult the switch arm 8G will first be actuated to tu 'rage the Contact 68 and then eneaee the contact rotate the rnezqcury switch Si to reclose the circuit therethrough.

In describing the operation oi' this system as a whole, it will be assumed that the parts are ln place.

the position shown in the single figure of the drawing. This is the cold position in which the room thermostat 51 is satisfied and the oil burner is deenergized. The room temperature will now begin to decrease until the resilient arm 59 engages contact 10, at which time no circuit will be closed. The room temperature will continue to decrease until the arm 66 engages contact 1l at which time a circuit will be established from the secondary winding 52 of the transformer 5I, conductor 12, contact 16, flexible arm 59, rigid arm 66, contact 1l, conductor 13, relay coil 66, conductor 14, switch arm 66, cold contact 61, conductor 15, electric heater 22, conductor 16 to the lever I5 and thence through the contacts 44 and 43 to the arm 42 on the reset member 4I,

and thence by means of conductor 11 back to the.

secondary winding 52. This will cause the relay 66 to pull in the switch arms 6l and 62 to engage their contacts 16 and 15. Closure of the contact 18 by the switch arm 6I will set up a holding circuit from contact lll, resilient arm 59, bimetallic element 56, conductor 86, contact 16, switch arm 6l and conductor 8l to the relay coil 60 so that on a rise in temperature the relay will not be dropped out when the arm 60 leaves the contact 1l but will remain energized until the resilient arm 59 leaves the contact 16. Engagement of the switch arm 62 with contact I9 will set up a circuit for the burner motor as follows: line wire 55, conductor 82, contact 16, switch arm 62, conductor 83, burner motor 63, conductors 84 and 85 back to the line 56. At the same time a circuit is established for the ignition means from the switch arm 62, conductor 86, mercury switch 69 which is in closed position, conductor 81, ignition means 64 and conductors 66 and 85 back to the line 56. This will simultaneously energize the burner motor and the ignition means. i

At this time it will be seen that on a call for heat by the room thermostat the burner motor, ignition means and electric heater 22 are simultaneously energized. Under normal conditions the fuel pumped to the furnace by' the burner motor will be ignited and combustiomwill take The heat of combustion will be felt by the thermal element of the combustion switch 65,

which will first cause the switch arm 66 to engage contact 68. This will establish a circuit from the secondary 52 through the room thermostat 51, relay coil to the switch arm 66 as traced above, and then to contact 68 and conductor 8S to the conductor 16 and thence back to the secondary 52 as traced above. It will be noted that this circuit does not pass through the electric heater 22 and hence the block v2I will not be heated to the point where the bar magnet will be released through the effect of the thermocouple 23 and windings 21 and 29. Through the response of the thermal element of the combustion device 65, switch arm 66 will next leave the contact 61 and the mercury switch 69 will then be moved to open circuit position to deenergize the ignition means 64. The burner motor will remain energized through the switch arm 62 and the system will now be in running position.

If the room thermostat later becomes satisfied, it breaks the circuit to the relay coil 60 and the switch arms 6I and 62 drop out to break all electric circuitsand the parts will return to the position as shown in the drawing. v

Assume 'now that the room thermostat has energized tive relay 66 and the burnerI motor 63,

the ignition means 64, and the electric heater 22 are energized as described above. If for some reason combustion should not take place, the switch arm 66 will remain in engagement with the contact 61 and hence the circuit will continue topass through the electric heater 22 which will, after a predetermined period of time determined by the size of the metallic block 2I, the size of the heater 22 and the current flow therethrough, raise the temperature of the block 2| to the point where the windings 21 and 29 reduce the effect of the permanent magnet I1 and the tensionspring I6 will rotate the lever I5 in a clockwise direction, breaking the circuit through the contacts 43 and 44. This will break the circuit to the relay coil 66 and the burner motor 63 and ignition means 64 will be deenergized.

The parts willy remain in this position until the' This mechanism will also shut the systemI down in case combustion should fail while the system is in running position. Assume for example that the room thermostat is calling for heat and that the system has been started as described above with the switch arm 66 engaging contact 68, the mercury switch 69 in open position and the burner motor 63 energized through the switch arm 62. If at this time combustion should fail, the thermal element of the combustion device will start to cool and immediately open the contact 68 which will drop out the relay 66 and shut the system down. The

room thermostat 51 will still be calling for heatv and as soon as the switch arm 66 engages the cold contact 61, the relay 60 will again be energized and the system will attempt to start. However, if the trouble is still present which caused the failure of combustion from running position, then combustion will not occur when the attempt is made to start and the electric heater 22 will remain energized and after a predetermined length of time bring the temperature of the metallic block 2I up to the point where the windings 21 and 29 will overcome the eifect of the magnet I1, permitting the spring I6 to break the circuit at the contacts 43 and 44 and shut v down the system as before Certain modifications and changes in the present construction will doubtless occur to those skilled in the art and it is thereby to be understood that I intend to be limited by the scope of appended claims and not by the speciic embodiment shown in the drawing and described in the specification.

I claim as my invention:

l.- In a device of the character described comprising in combination, a pivoted lever, a movable contact mounted on said lever, a stationary contact positioned for cooperation therewith, a spring tending to rotate said lever in a direction to open said contacts, a normally unmagnetized core, a permanent magnet carried by said lever and lying adjacent said core to hold said contacts in closed position against the action of said spring, a thermocouple having a hot junction and a cold junction, lagging for said said hot junction, an electrical conductor in circuit with said. thermocouple and wound around said core, and a heater for said hot junction, said cold. junction responding to ambient temperature, said thermocouple producing a current iiow through said winding which will reduce the effect of said permanent magnet to a point where said spring can open the contacts when the heater has raised the temperature of said hot junction to a predetermined degree.

2. In a device of the character described cornprising in combination, a pivoted lever, a movable contact mounted on said lever, a stationary con tact positioned for cooperation therewith, a spring tending to rotate said lever in a direction to open said contacts, a normally unmag'netized core, a permanent magnet carried by said lever and lying adjacent said core to hold said contacts in closed position against the action of said spring, a tliermocouple having a hot junction and a cold junction, lagging for said hot junction, an electrical conductor in circuit with said thermocouple and Wound around said core, a heater for said hot junction, said cold junction responding to ambient temperature, said thermocouple pron ducing a current fiow through said winding which will reduce the effect of said permanent magnet to a point where said spring can open the contacts when the heater has raised the temperature of said hot junction to a predetermined degree, and means for manually returning said lever to its original position, said means being incapable of closing said contacts unless said permanent magnet is capable of preventing said spring from rotating said lever.

3. In a device of the character described comprising in combination, a pivoted lever, a movable contact mounted on said lever, a stationary contact positioned for cooperation therewith, a

spring tending to rotate said lever in a direction to open said contacts, a normally unmagnetized core, a permanent magnet carried by said lever and lying adjacent said core to hold said conn tacts in closed position against the action of said spring, a therrnocouple having a hot junction and a cold junction, lagging for said hot junction, an electrical conductor in circuit with said thermocouple and wound around said core, a heater for said hot junction, said cold junction responding to ambient temperature, said thermocouple producing a current flow through said winding which will reduce the effect of said permanent magnet to a point where said spring can open the contacts when the heater has raised the tem.. perature of said hot junction to a predetermined degree, and means for manually returning said lever to its original position, said means being incapable of closing said contacts unless said permanent magnet is capable of preventing said spring from rotating said lever, and adjustable means for limiting the movement of said permanent magnet toward said core. l

4. In a device of the character described, a lever pivoted intermediate its ends, a stationary contact, a movable contact mounted on one end of said lever. for cooperation with said stationary' contact, a first member mounted on the other end of said lever, a second stationary member,

CII

andere said members being magnetically attracted toward each other to hold said contacts in closed position, spring means tending to rotate said lever in a direction to open said contacts, a heat absorbing block, a heater for said block, a thermocouple whose hot junction is responsive to the temperature of said block. and whose cold june tion is responsive to ambient temperature, and an electric winding in series with said thermo couple, said winding being adapted to neutralize said magnetic attraction between said members when said block is heated to a predetermined temperature.

5, In a device of the character described com prising in combination, a switch biased to one position of operation, a polarized relay normally retaining said switch in its other position of opL- eration, a heat absorbent mass, a thermocouple having a hot junction and a cold junction, said hot junction being responsive to the temperature of said mass, a winding associated with said relay and in series with said thermocouple, and means for heating said mass to a temperature sufcient to cause said thermocouple to develop a ilow of electricity through said winding to over come the effect of said relay and permit said switch to move to its said one position under the influence of its bias.

6. In a device of the character described comprising in combination, a switch, a switch operating member biased to switch open position, magnetic mea-ns normally retaining said member in switch closed position, a thermocouple having hot and cold junctions, said hot junction being lagged and said cold junction responding to ambient temperature, a winding in series with said thermocouple and associated with said magnetic means in such a manner that a predetermined .ilow of electricity through said winding will overcome the holding effect of said magnetic means, and means for heating said lagged hot junction whereby said thermocouple will produce said predetermined flow of electricity and said switch operating member will be released to open said switch.

7. In a device of the character described comprising in combination, a switch, a switch oper-w ating member biased to switch open position, magnetic means normally retaining said mem-i ber in switch closed position, a thermocouple having hot and cold junctions, said hot junction beingr lagged and said cold junction responding to ambient temperature, a winding in series with said thermocouple and associated with said magnetic means in such a manner that a predeter mined ilow of electricity through said winding will overcome the holding effect of said magnetic means, means for heating said lagged hot june tion whereby said therrnocouple will produce said predetermined iiow of electricityA and said switch operating member will be released to open said switch, and a trip-free manual reset for said, switch whereby said switch can be closed man ually only ii" the hot junction of said thermocouple is suilciently cool so that said magnetic means retains said switch operating member in switch closed position.

HUBERT T. SPARROW. 

